$ { \left[\begin{array}{r}{1} \\ {-1} \\ {-2}\end{array}\right]+\left[\begin{array}{r}{2} \\ {3} \\ {1}\end{array}\right] = {?} } $
To find the sum of two matrices, add the corresponding items between each matrix. ${\left[\begin{array}{r}{1}+{2} \\ {-1}+{3} \\ {-2}+{1}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{r}{3} \\ {2} \\ {-1}\end{array}\right]}$